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Mycobacterium tuberculosis . Light micrograph of the rodlike bacteria Mycobacterium tuberculosis, causative agent of tuberculosis in humans. These bacteria may occur as filamentous branching organisms. They are inhaled into the lungs where they form a primary tubercle (nodular lesion). They then spread to the nearest lymph nodes. This form is called pulmonary tuberculosis or consumption & phthisis (wasting). Many people are infected without showing symptoms, the body's immune system holds the disease in check. They remain carriers, transmitting the bacilli by coughing or sneezing. Infected cow's milk may also transmit the bacillus. Mag: x330 at 35mm size.